This invention relates to the conversion of a waste product from the aluminum industry into a material which has substantial economic value.
In the production of alumina by the Bayer process, up to two tons of a filter-wet residue having a water content of from 40 to 50% by weight may be obtained for every ton of alumina produced, and it has been estimated that more than 5 million tons of such residue accumulate annually.
A general description of the Bayer alumina process may be found in the MCGRAW-HILL ENCYCLOPEDIA OF SCIENCE AND TECHNOLOGY, McGraw-Hill Book Co., N.Y. (1966), Vol. 1, pages 290-292, under the heading "Alumina Extraction." Briefly, this is a continuous process wherein the bauxite ore is extracted with a leaching liquor containing 280g NaO per liter at a temperature of about 180.degree. C and a pressure in the order of 8 atmospheres. The insoluble residue is separated from the crude extract, usually on a rotary drum filter.
This residue is a deeply-colored clay-like substance which is highly hydrated (about 35-50% water by weight), the mineral content of which is principally compounds of iron and aluminum with lesser proportions of alkalis and compounds of other metals. Because of its color and consistency, this material is commonly referred to as "red mud."
The proportions of the mineral components in red mud vary somewhat with the particular bauxite ore from which it is derived. The following Table, based upon analyses of red muds dried at 105.degree. C, is indicative of these compositional variations.
Table 1 ______________________________________ Al.sub.2 O.sub.3 15 - 25 (% by weight) Fe.sub.2 O.sub.3 20 - 55 " TiO.sub.2 5 - 10 " SiO.sub.2 8 - 12 " CaO 1 - 4 " Na.sub.2 O 5 - 12 " loss at red heat 7 - 10 " ______________________________________
The substantial amounts of such residue from the production of alumina has posed a substantial ecological as well as an economic problem for many years and various proposals have been made for the industrial utilization of this otherwise waste product, none of which have proven satisfactory.
One such proposal is represented by DAS (German Published Patent Application) No. 1,592,104 according to which red mud is partially decomposed by treating with 60-96% aqueous sulfuric acid in less than stoichiometric quantities, calculated with respect to the metal oxides in the mud. It is suggested that such a product can be used in various fields, for example in the treatment of raw water and various industrial waste waters and, particularly in combination with humus fertilizer and ammonium sulfate or other fertilizers, for improving the structure of the soil.
The method of DAS No. 1,592,104 is not suitable, however, for the manufacture of flocculating agents on an industrial scale. For example, the moisture content of the red mud must be reduced to about 10-20% before the acid treatment, as otherwise the reaction product is not dry enough for satisfactory storage. Moreover, the predried red mud must be finely ground with concomitant production of considerable dust which is objectionable to the workers and the surrounding community. In addition, the dried and ground red mud is difficult to meter out accurately and even with highly efficient mixers it does not mix completely and uniformly with the sulfuric acid.
A further drawback is that the reaction mixture is initially in a pasty state and must be transferred to special equipment for cooling to a solid product which is suitable for crushing and grinding to the required grain size. During grinding, additional difficulties arise because the reaction product tends to stick to the grinding machinery. This same problem occurs during the screening of the ground material.
These problems obviously not only complicate large scale operations according to DAS No. 1,592,104 but substantially add to the operating costs for labor and equipment.